Respiratory Substrates and Quotient

• Respiratory Substrates

  • Respiratory substrate: Organic molecule for respiration to release energy + make ATP (e.g. glucose)
  • All food molecules converted to enter cellular respiration pathway
  • Some at glycolysis, others Krebs cycle
  • All catabolic pathways for carbs, proteins + lipids connect
  • Metabolic pathways are porous (not closed; many pathway's products are reactants in others)
  • Oxidative phosphorylation make most ATP; substrate with more H atoms per mole inc ATP but need more O_2 per substrate mole

  Respiratory Quotient

  • O_2 vol in + CO_2 vol out measured to calc respiratory quotient (RQ); CO_2 vol/O_2 vol
  • RQ values: carb 1.0, protein 0.8-0.9 + fat 0.7
  • RQ value of most animals at rest is range 0.8-0.9 (human: ~0.85)
  • Necessary view data w/ caution, may be mixture; 0.8 RQ could point to both protein + fat used
  • RQ >1, indicates anaerobic respiration occurring; more CO_2 produced than O_2 consumed
• Carbs

  • Glucose: Main respiratory substrate (brain cells only respire this)
  • Other hexose sugars (fructose + galactose) modified for glycolysis
  • Animal cells store glycogen, plant cells starch
  • Theoretical energy yield 2870kJmol-1; 94 ATP per glucose (ADP + Pi makes 30.6kJmol-1)
  • Actual yield ~30 molecules; 32% efficiency, remaining lost as heat
• Proteins

  • Excess AA are deaminated in liver
  • Keto acids produced to metabolise to glycogen/fat for storage (need ATP; reduces net production)
  • When fasting or in vigorous exercise, muscle tissue can be respired
  • Some AA converted to pyruvate, some to acetate + some enter the Krebs cycle directly
  • 1 molecule has slightly more H than 1 glucose molecule; so little more ATP produced per protein molecule
• Lipids

  • Key respiratory substrate, partic mammalian skeletal muscle
  • Triglycerides are hydrolysed to form fatty acids + glycerol; converted to pyruvate + respired
  • Fatty acids have long hydrocarbon chains, so many H; proton source for chemiosmosis
  • Each combines w/ CoA, energy from hydrolysis ATP -> AMP + 2Pi
  • Fatty acid-CoA complex enter mitochondrial matrix where 2C acetate groups are successively removed + combined w/ CoA
  • β-oxidation path of long chain fatty acid produces many reduced NAD + FAD